The invention is concerned with increasing the portion of heavy petroleum crudes which can be utilized as catalytic cracking feedstock to produce premium petroleum products, particularly motor gasoline of high octane number. The heavy ends of many crudes are high in Conradson Carbon and metals which are undesirable in catalytic cracking feedstocks. The present invention provides an economically attractive method for utilizing the residues of atmospheric and vacuum distillations, commonly called atmospheric and vacuum residue or "resids." The undersirable CC (for Conradson Carbon) and metal bearing compounds present in the crude tend to be concentrated in the resids because most of them are of high boiling point.
When catalytic cracking was first introduced to the petroleum industry in the 1930's, the process constituted a major advance in its advantages over the previous technique for increasing the yield of motor gasoline from petroleum to meet a fast-growing demand for that premium product. The catalytic process produces abundant yields of high octane naphtha from petroleum fractions boiling above the gasoline range, upwards of about 400.degree. F. Catalytic cracking has been greatly improved by intensive research and development efforts and plant capacity has expanded rapidly to a present-day status in which the catalytic cracker is the dominant unit, the "workhorse" of a petroleum refinery.
As installed capacity of catalytic cracking has increased, there has been increasing pressure to charge to those units greater proportions of the crude entering the refinery. Two very effective restraints oppose that pressure, namely Conradson Carbon and metals content of the feed. As these values rise, capacity and efficiency of the catalytic cracker have been adversely affected.
The effect of higher Conradson Carbon is to increase the portion of the charge converted to "coke" deposited on the catalyst. As coke builds up on the catalyst, the active surface of the catalyst is masked and rendered inactive for the desired conversion. It has been conventional to burn off the inactivating coke with air to "regenerate" the active surfaces, after which the catalyst is returned in cyclic fashion to the reaction stage for contact with and conversion of additional charge. The heat generated in the burning regeneration stage is recovered and used, a least in part, to supply heat of vaporization of the charge and endothermic heat of the cracking reaction. The regeneration stage operates under a maximum temperature limitation to avoid heat damage of the catalyst. Since the rate of coke burning is a function of temperature, it follows that any regeneration stage has a limit of coke which can be burned in unit time. As CC of the charge stock is increased, coke burning capacity becomes a bottleneck which forces reduction in the rate of charging feed to the unit. This is in addition to the disadvantage that part of the charge has been diverted to an undesirable reaction product.
Metal bearing fractions contain, inter alia, nickel and vanadium which are potent catalysts for production of coke and hydrogen. These metals, when present in the charge, are deposited on the catalyst as the molecules in which they occur are cracked and tend to build up to levels which become very troublesome. The adverse effects of increased coke are as reviewed above. The lighter ends of the cracked product, butane and lighter, are processed through fractionation equipment to separate components of value greater than fuel to furnaces, primarily propane, butane and the olefins of like carbon number. Hydrogen, being incondensible in the "gas plant," occupies space as a gas in the compression and fractionation train and can easily overload the system when excessive amounts are produced by high metal content catalyst, causing reduction in charge rate to maintain the FCC unit and auxiliaries operative.
These problems have long been recognized in the art and many expedients have been proposed. Thermal conversions of resids produce large quantities of solid fuel (coke) and the pertinent processes are characterized as coking, of which two varieties are presently practiced commercially. In delayed coking, the feed is heated in a furnace and passed to large drums maintained at 780.degree. to 840.degree. F. During the long residence time at this temperature, the charge is converted to coke and distillate products taken off the top of the drum for recovery of "coker gasoline," "coker gas oil" and gas. The other coking process now in use employs a fluidized bed of coke in the form of small granules at about 900.degree. to 1050.degree. F. The resid charge undergoes conversion on the surface of the coke particles during a residence time on the order of two minutes, depositing additional coke on the surfaces of particles in the fluidized bed. Coke particles are transferred to a bed fluidized by air to burn some of the coke at temperatures upwards of 1100.degree. F., thus heating the residual coke which is then returned to the coking vessel for conversion of additional charge.
These coking processes are known to induce extensive cracking of components which would be valuable for catalytic cracking charge, resulting in gasoline of lower octane number (from thermal cracking) than would be obtained by catalytic cracking of the same components. The gas oils produced are olefinic, containing significant amounts of diolefins which are prone to degradation to coke in furnace tubes and on cracking catalysts. It is often desirable to treat the gas oils by expensive hydrogenation techniques before charging to catalytic cracking. Coking does reduce metals and Conradson Carbon but still leaves an inferior gas oil for charge to catalytic cracking.
Catalytic charge stock may also be prepared from resids by "deasphalting" in which an asphalt precipitant such as liquid propane is mixed with the oil. Metals and Conradson Carbon are drastically reduced but at low yield of deasphalted oil.
Solvent extractions and various other techniques have been proposed for preparation of FCC charge stock from resids. Solvent extraction, in common with propane deasphalting, functions by selection on chemical type, rejecting from the charge stock the aromatic compounds which can crack to yield high octane components of cracked naphtha. Low temperature, liquid phase sorption on catalytically inert silica gel is proposed by Shuman and Brace, OIL AND GAS JOURNAL, Apr. 16, 1953, Page 113.
Of the types of catalytic cracking systems, the one of the greatest present interest is Fluid Catalytic Cracking (FCC). The installed plants of this type are characteristically large, and usually designed to process from about 5,000 to 135,000 bbls/day of fresh feed. Briefly, the catalyst section of the plant consists of a cracking section where a heavy chargestock is cracked in contact with fluidized cracking catalyst, and a regenerator section where fluidized catalyst coked in the cracking operation is regenerated by burning with air. All of the plants utilize a relatively large inventory of cracking catalyst which is continuously circulating between the cracking and regenerator sections. The size of this circulating inventory in existing plants is within the range of 50 to 600 tons, the newer plants being designed for short time riser cracking with smaller catalyst inventory than that in older plants. Because the catalytic activity of the circulating inventory of catalyst tends to decrease with age, fresh makeup catalyst usually amounting to about one to two percent of the circulating inventory, which corresponds to about 0.1 to 0.25 lbs. per bbl. of fresh feed, is added per day to maintain optimal catalyst activity, with daily withdrawal plus losses of about like amount of aged circulating inventory, commonly referred to as "equilibrium" catalyst. The considerations which are involved in setting catalyst make-up policy are adequately reviewed in "Dynamic Optimization of Catalyst Make-Up Rate for Catalytic Cracking Systems" W. Lee, Ind. Eng. Chem. Process Des. Development, Vol. 9, No. 1, pp. 154-158 (Jan. 1970). That article provides equations of state and an algorithm for optimizing the make-up rate. The same is hereby incorporated by this reference.
In general, the oils fed to this process are principally the petroleum distillates commonly known as gas oils, which boil in the temperature range of about 650.degree. F. to 1000.degree. F., supplemented at times by coker gas oil, vacuum tower overhead, etc. These oils generally have an API gravity in the range of about 15 to 45 and are substantially free of metal contaminants.
The chargestock, which term herein is used to refer to the total fresh feed made up of one or more oils, is cracked in the reactor section in a reaction zone maintained at a temperature of about 800.degree. F. to 1200.degree. F., a pressure of about 1 to 5 atmospheres, and with a usual residence time for the oil of from about one to ten seconds with a modern short contact time riser design. The catalyst residence time is from about one to fifteen seconds. The cracked products are separated from the coked catalyst and passed to a main distillation tower where separation of gases and recovery of gasoline, fuel oil, and recycle stock is effected.
Petroleum refiners usually pay close attention in the fluid catalytic cracking process (hereinafter referred to as the FCC process) to supplying feedstocks substantially free of metal contaminants. The reason for this is that the metals present in the chargestock are deposited along with the coke on the cracking catalyst. Unlike the coke, however, they are not removed by regeneration and thus they accumulate on the circulating inventory. The metals so deposited act as a catalyst poison and, depending on the concentration of metals on the catalyst, more or less adversely affect the efficiency of the process by decreasing the catalyst activity and increasing the production of coke, hydrogen and dry gas at the expense of gasoline and/or fuel oil. Excessive accumulation of metals can cause serious problems in the usual FCC operation. For example, the amount of gas produced may exceed the capacity of the downstream gas plant, or excessive coke loads may result in regenerator temperatures above the metallurgical limits. In such cases the refiner must resort to reducing the feed rate with attendant economic penalty. Thus, a catalyst inventory that contains excessive deposits of metal is normally regarded as highly undesirable.
The principal metal contaminants in crude petroleum oils are nickel and vanadium, although iron and small amounts of copper also may be present. Additionally, trace amounts of zinc and sodium are sometimes found. It is known that almost all of the nickel and vanadium in crude oils is associated with very large nonvolatile hydrocarbon molecules, such as metal porphyrins and asphaltenes. Crude oils, of course, vary in metal content, but usually this content is substantial. An Arab light whole crude. for example, may assay 3.2 ppm (i.e. parts by weight of metal per million parts of crude) of nickel and 13 ppm of vanadium. A typical Kuwait whole crude, generally considered of average metals content, may assay 6.3 ppm of nickel and 22.5 ppm of vanadium. Regardless of the crude source, however, it is known that distillates produced from the crude are almost free of the metal contaminants which concentrate in the residual oil fractions.
Petroleum engineers concerned with the FCC process have several ways for referring to the metal content of a chargestock. One of these is by reference to a "metals factor", designated F.sub.m. The factor may be expressed in equation form as follows: EQU F.sub.m =ppm Fe+ppm V+10 (ppm Ni+ppm Cu)
A chargestock having a metals factor greater than 2.5 is considered indicative of one which will poison cracking catalyst to a significant degree. This factor takes into account that the adverse effect of nickel is substantially more than that of vanadium and iron present in equal concentrations with the nickel.
Another way of expressing the metals content of a chargestock is as "ppm Nickel Equivalent" which is defined as EQU ppm Nickel Equivalent=ppm nickel+0.25 ppm vanadium
For the purpose of this specification, the value of ppm Nickel Equivalent will be used in discussing metals content of metal-contaminated oils, distillate stocks, and catalysts. As shown above, no mention is made of copper because this metal usually is not present to any significant extent. However, it is to be understood herein that if it is present in significant concentration, it is to be included in the computation of Nickel Equivalent and weighted as nickel.
It is current practice in FCC technology to control the metals content of the chargestock so that it does not exceed about 0.25 ppm Nickel Equivalent. Catalyst make-up is managed to control the activity of the circulating inventory. With this practice, for example, in a plant utilizing 50,000 bbl/day of fresh feed, and an equilibrium catalyst withdrawal of 9 tons per day, the withdrawn catalyst under steady state conditions will contain about 300 ppm Nickel Equivalent of metals, taking into account that the fresh catalyst contributes 70 ppm to this value. Thus, the circulating inventory is maintained at about 300 ppm Nickel Equivalents of metal, which is considered tolerable, the usual range being at about 200 to 600 ppm, with preferred operation being at about 200 to 400 ppm. It is to be understood, of course, that the metals content of the chargestock may vary from day to day without serious disruption, provided that the weighted average of the metals content does not exceed about 0.25 ppm nickel equivalent of metal.
It is important, for the purpose of the present invention, to understand that all references to the metals content of an oil, or of a chargestock, refer to the time-weighted average taken over a substantial period of time such as one month, for example. Because of the large inventory of catalyst relative to the total metals introduced into the system by the chargestock in one day, for example, the metals content of the catalyst changes little each day with fluctuations in the quality of the chargestock. However, a persistent increase in the metals content of the latter will in time result in a well-defined, calculatable increase in the metals content of the circulating inventory of catalyst, which determines the performance of the FCC unit. In fact, it is evident that the calculating inventory of catalyst, by its metals content, provides a time-average value of the metals content of the chargestock. It is in this context, then, that the phrase "metals content of the chargestock" is used herein.
For the purpose of this invention, chargestocks to the FCC process that contain up to about 0.40 ppm Nickel Equivalent of metal contaminants will be regarded as substantially free of metal contaminants. Chargestocks that contain at least about 0.50 ppm Nickel Equivalents of metal will include those chargestocks referred to as metal-contaminated.
The effects of nickel, vanadium and other heavy metals on activity and selectivity of FCC catalysts are discussed in detail by Cimbalo, Foster and Wachtel in a paper presented at the 37th midyear meeting of the API Division of Refining under the title "Deposited Metals Poison FCC Catalyst" and published at pages 112-122 of the Oil and Gas Journal for May 15, 1972, the full contents of which are incorporated herein by reference. Those authors show that metal contaminants of cracking catalyst decline in poisoning activity through repeated cycles of oxidation and reduction and propose a value of "effective metals" determined by multiplication of actual metal concentration by a fraction related to the rate of fresh catalyst make-up as percent of catalyst inventory. Although the authors note that different cracking catalysts may respond differently to metal poisoning and that differences in operation of the regenerator may affect rate of metal deactivation, they establish a single standard for determination of "effective metal" values to be applied generally, presumably having regard to specific catalyst and operating conditions.
The residual fraction of single stage atmospheric distillation or two stage atmospheric/vacuum distillation also contains the bulk of the crude components which deposit as resinous or tar-like bodies on cracking catalysts without substantial conversion. These are frequently referred to as "Conradson Carbon" from the analytical technique of determining their concentration in petroleum fractions. The Cimbalo article above cited classifies coke on spent catalyst in four groups: catalytic coke resulting from cracking of charge components; cat-to-oil, related to reactor stripper efficiency; carbon residue (Conradson) as just discussed; and contaminant coke derived from dehydrogenation reactions promoted by the heavy metal poisons nickel, vanadium, etc. The residual stocks not only provide metal poisoning of the catalyst but also show high Conradson Carbon values which are reflected by coke of that class very nearly equal to the Conradson Carbon number. It will be seen that the increment of Conradson Coke results from deposition on the catalyst of non-volatile hydrocarbons in the charge without significant change in nature of the deposited hydrocarbons.
With very limited exceptions, residual oils have not been successfully included in the chargestocks to the FCC process. The reasons for this are not fully understood, although from the foregoing discussion it is apparent that their high metals content is certainly a major contributing factor, as is the typically high Conradson Carbon. There has been interest in using them, however. The reason for this interest becomes apparent when we consider, for example, that typically only about 26 volume % of an Arab light whole crude is the 650.degree.-1000.degree. F. gas oil fraction, while the total 650.degree. F. plus resid constitutes about 43 volume %. Thus, were it feasible to efficiently operate with residual oil fractions, a very substantial increase in the amount of gasoline plus fuel oil derivable from a barrel of crude could be obtained. In some refineries, the vacuum resid remaining after the distillation of the gas oil is coked and the coker gas oil is included in the FCC chargestock. However, it is generally recognized that coker gas oil, because of its high unsaturated and high aromatics content, is a poor quality feed.
It has been proposed in the prior art to hydrotreat residual oils under such conditions that the metals content is brought into the range commonly associated with gas oils. Such hydrotreated residual oils, substantially free of metal contaminants, may then be used as chargestock or a component thereof for the FCC process. Processes to achieve such metals and sulfur reduction are disclosed in U.S. Pat. No. 3,891,541, issued June 24, 1975 and U.S. Pat. No. 3,876,523, issued Apr. 8, 1975, for example, the entire contents of which are incorporated herein by reference. The combination of hydrotreating to reduce metals and sulfur content followed by cracking also is disclosed in a publication by Hildebrand et al. in The Oil and Gas Journal, pp 112-124, Dec. 10, 1973, the entire contents of this article being incorporated herein by reference. However, no installation is known which has adopted the proposed scheme, probably because the cost and severity associated with the operation involves a heavy economic penalty.
The concurrent problems of heavy metal and Conradson Carbon content of heavy stocks have been approached by the expedient of catalyst modification. U.S. Pat. No. 3,944,482 proposes a cracking catalyst of active aluminosilicate zeolite dispersed in a matrix of large pore refractory inorganic oxide. The patentee suggests that the tendency of the metals to deposit in large pore structures renders the matrix a sacrificial component which protects the active zeolite cracking surfaces of the zeolite from metal contamination. The effectiveness of large pore structures in adsorbing and/or converting metal bearing components of crude is widely recognized. Many hydrotreating catalysts are preferably prepared by deposit of a Group VI metal with nickel or cobalt on a large pore alumina or the like. See also U.S. Pat. No. 3,947,347 on demetallizing petroleum fractions in admixture with hydrogen over a large pore catalyst without added hydrogenation metal catalysts and U.S. Pat. No. 2,472,723 and 4,006,077.
Whether or not the charge stock contains heavy metals, activity of the catalyst added as make up has a profound effect on operation of an FCC Unit and is an important factor considered by the refiner in order to accomplish his objectives. It is usual to consider cracking catalysts in terms of capability to produce gasoline. This takes no account of the very significant proportion of catalytic cracking capacity in refineries producing only minor gasoline yields. In a market having a small demand for gasoline as compared with the demand for light distillate fuels (No. 2 heating oil, jet fuel, diesel fuel, kerosene) FCC Units are operated to minimize gasoline and maximize the distillates boiling above gasoline. Such units will generally employ a catalyst of relatively low activity. To meet the demand for catalysts of various activity levels, catalyst manufacturers stand prepared to deliver different grades of catalyst over a range of activities. One way to accomplish this without conducting manufacturing operations in accordance with a large number of schemes is to manufacture one or a few different catalysts of differing activity. Intermediate grades are conveniently achieved by blending substantially inert particles of like fluidization properties with an active fluid catalyst to thereby provide a total catalyst of lower activity than the active portions. By all these techniques including blending as in U.S. Pat. No. 2,455,915 and the inert, large pore matrix of U.S. Pat. No. 3,944,482, the refiner has at hand a catalyst of fixed activity which he adds to his unit in order to maintain an equilibrium activity according to the equation: ##EQU1## where A.sub.F is activity of fresh catalyst
A.sub.E is equilibrium activity of the total catalyst inventory in the unit PA0 S is the rate of make-up in percentage of total inventory per day PA0 K is a constant representing rate of catalyst deactivation
This is essentially equation (15) of the Lee article cited above, which see for derivation and more detailed explanation of terms. It will be apparent that activity is a relative term, the absolute value of which is dependent on the test procedure. In this specification, "activity" refers to the value determined by a microactivity test (MAT) conducted by cracking a Mid-Continent Gas Oil of the following properties:
______________________________________ Gravity, .degree.API 27.9 CCR, Wt. % 0.23 Sulfur, Wt. % 0.6 Initial Boiling Point, .degree.F. 482 50% Point 749 90% Point 979 ______________________________________
The cracking test is conducted by contacting 1.2 grams of the gas oil with 6.0 grams of catalyst (c/o=5) at 910.degree. F. and a feed time of 96 seconds for WHSV of 7.5. The liquid product is distilled and "conversion" is reported as 100 minus weight percent based on feed of liquid product boiling above 421.degree. F. Activity is then calculated as ##EQU2##
Methods are known to the art for preparing cracking catalysts of very low activities such as severely steamed amorphous silica-alumina of activity at 1 or less to very high activities above 20 such as highly active aluminosilicate zeolites. See U.S. Pat. No. 3,493,519. Such catalysts of very high activity have not come to the market because existing equipment for catalytic cracking is not capable of utilizing the activity, hence no refiner will pay the higher price which must be charged in view of the high production cost.
In summary, the refiner who presently wishes to charge residual stocks is compelled to adjust his operations to the options available to him. The catalyst make-up rate to his catalytic cracker is determined by the activity considerations spelled out in the Lee article. To avoid adverse effects of metals deposited on the catalyst, the refiner hydrotreats the resid, sends it to a coker or deasphalter or lives with the problem of metal on cracking catalyst of whatever fresh activity he selects.